Image-forming apparatus such as electrophotographic apparatus and electrostatic recording apparatus, in which a prescribed latent image on a photosensitive drum is rendered visible, are generally equipped with a cleaning device for removing a developer remaining on the photosensitive drum.
For example, an image-forming apparatus of such a construction as schematically illustrated in section in FIG. 1 is generally used. This apparatus is provided with a photosensitive drum (1), a cleaning device (2) equipped with a cleaning blade (2a) disposed about the photosensitive drum, a charging device (3), an exposing device (4), a developing device (5), a development roll (8), a transferring device (6) and the like. The surface of the photosensitive drum (1), which has been charged to a desired extent by the charging device (3), is selectively exposed by the exposing device (4), thereby forming a latent image. An area of the latent image is developed with a developer (toner) by making use of the developing device (5). A toner image thus developed is transferred to a transfer paper sheet (7) by the transferring device (6). After completion of the transfer process, the toner remaining on the photosensitive drum (1) is removed by the cleaning blade (2a) of the cleaning device (2).
However, the above-described apparatus involves a problem that since the remaining toner accumulated in the cleaning device must be discarded, its maintenance is complicated, and pollution of the circumference of the apparatus and environment is brought about. In addition, the photosensitive drum is worn due to the friction with the cleaning blade, so that the image-forming characteristics and life of the image-forming apparatus are lowered. Furthermore, the provision of the cleaning device results in reduction in degree of freedom in its design.
In order to solve such problems, it has been proposed to use a one-component developer and conduct development and cleaning at the same time by the same developing device (Japanese Patent Application Laid-Open Nos. 203182/1987 and 7972/1991).
A schematic cross-sectional view of an exemplary image-forming apparatus for achieving such a process is illustrated in FIG. 2. Incidentally, in FIG. 2, the character (9) indicates a layer-thickness regulator for toner, and other characters designate members or devices respectively corresponding to those of the same characters in FIG. 1.
In the process making use of the apparatus illustrated in FIG. 2 to conduct development and cleaning at the same time, the toner remaining on the photosensitive drum after completion of a transfer process is recovered within the developing device according to the following principle. Namely, a surface potential of an unexposed area (non-latent-image area) and a surface potential of an exposed area (latent-image area) of the photosensitive drum (1), and a development bias voltage applied to the development roll (8) are supposed to be Vo, Vq and Vb, respectively, and a surface potential Ve of the development roll (8) is assumed to be identical with the development bias voltage Vb. The electrostatic latent image formed on the photosensitive drum is developed in reversal fashion with a one-component developer (toner) charged to the same polarity as the electric charge of the latent image.
In this reversal development, the above-described surface potentials are preset so as to satisfy the following relationship: EQU .vertline.Vo.vertline.&gt;.vertline.Ve.vertline.&gt;.vertline.Vq.vertline.
wherein Vo, Ve and Vq have the same polarity. In the latent-image area on the photosensitive drum, force toward the photosensitive drum is exerted on the toner on the development roll by a potential difference .vertline.Ve-Vq.vertline., thereby conducting development. After completion of the transfer process, force toward the development roll is exerted on the toner remaining on the non-latent-image area by a potential difference .vertline.Vo-Ve.vertline., thereby recovering the remaining toner, i.e., cleaning the photosensitive drum. According to this simultaneous process of development and cleaning, the conventional cleaning device becomes unnecessary.
In this development process, a toner layer uniform and thin in thickness is also formed on the development roll (8) by the layer-thickness regulator for toner (9). Further, as a toner, there is used a non-magnetic one-component developer comprising a binder resin and a colorant, containing no magnetic powder and having a high specific resistance.
It was however found from the results of an experiment by the present inventors that when a potential difference .vertline.Ve-Vq.vertline. is made great with a view toward attaining a sufficient image density, a potential difference .vertline.Vo-Ve.vertline. required to recover the remaining toner becomes small, so that cleaning becomes incomplete and the formation of ghost images occurs. On the contrary, when the potential difference .vertline.Vo-Ve.vertline. is made great with a view toward improving the cleaning ability, the potential difference .vertline.Ve-Vq.vertline. required for the development becomes small, so that it is impossible to attain a satisfactory image density. If the transferability of the toner on the photosensitive drum to transfer paper is poor and the toner hence remains thereon to a great extent, it is necessary to properly control the respective surface potentials Vo, Ve and Vq and moreover to regulate the thickness of a toner layer formed on the development roll and the rotational ratio of the photosensitive drum to the development roll so as to keep the amount of the toner required for the development proper, thereby making the amount of the toner remaining on the photosensitive drum after completion of the transfer small, in order to satisfy both image density and cleaning ability. The latitude of these proper conditions is extremely limited, so that it is difficult to properly control them.